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The most common current signal standard in modern use is the 4 to 20 milliamp (4-20 mA) loop, with 4 milliamps representing 0 percent of measurement, 20 milliamps representing 100 percent, 12 milliamps representing 50 percent, and so on. A convenient feature of the 4-20 mA standard is its ease of signal conversion to 1-5 volt indicating instruments. A simple 250 ohm precision resistor connected in series with the circuit will produce 1 volt of drop at 4 milliamps, 5 volts of drop at 20 milliamps,The current loop scale of 4-20 milliamps has not always been the standard for current instruments: for a while there was also a 10-50 milliamp standard, but that standard has since been obsoleted. One reason for the eventual supremacy of the 4-20 milliamp loop was safety: with lower circuit voltages and lower current levels than in 10-50 mA system designs, there was less chance for personal shock injury and/or the generation of sparks capable of igniting flammable atmospheres in certain industrial environments.
Aside from the fact that you can easily diagnose the system if there is a break in the cables which resulted to a 0 mA reading, the 4mA is used to power the instruments especially if they are passive devices. In addition to that, the 4-20mA is not affected by the voltage drop due to resistance of the cable as opposed to 1-5V signalling.
4ma means live zero and 4ma is equal to 1 Volt .for long distance voltage will be drop but ma signal will not be droped.
4 - 20 is for Hart.
currently I see the use of Filedbus in increasing in the industry which using differed principle as Hart (4 to 20)
0-3 mA doesn't provide proper output for field instruments I is unstable
4- ma is the best linearity.
0-3 ma is not suitable for instruments
Basically because of the popularity of HART Protocol
because the transmitter consists of a resistance of ohm with voltage range1-5 volt , that means the current range is4- mA according to OHM law
It is the ideal range for all kinds of conductors for conduction of electricity.Especially semi conductors which has a linear charactristics between voltah=ge and curreent in the range of 4-20 ma
For Industrial control instruments, 4– mA current loops are commonly used for analog signaling, with4 mA representing the lowest end of the range and mA the highest.
It is a very simple and cost effective option to connect and configure, since it uses less wiring than other signals, hence reduces initial setup costs.
It is easy to detect a fault in the system.
It is a good option for traveling long distances, and also less sensitive to electrical noise.
